1. Field of the Invention
The present invention relates to a photomask for near-field exposure, a near-field exposure method, and a near-field exposure apparatus with which a minute pattern can be formed.
2. Related Background Art
As semiconductor memories are increasing in capacity, and CPU processors are enhanced in processing speed and level of integration, further miniaturization of photolithography is becoming indispensable. In general, the ability of a photolithography apparatus for fine processing is limited by the wavelength of light used by the apparatus.
With the trend being that light having a shorter wavelength is used in photolithography apparatuses, ultraviolet lasers are now employed, allowing fine processing on the order of 0.1 μm.
After all this advancement in miniaturization, photolithography still has many problems to clear up in order to achieve fine processing of 0.1 μm or finer. For instance, further shortening of the laser wavelength and development of a lens that works for that shorter wavelength range are required.
There has been proposed, as a measure for allowing optical fine processing of 0.1 μm or finer, a fine processing apparatus that uses the structure of a scanning near-field optical microscope (hereinafter, abbreviated as SNOM). For example, the apparatus uses evanescent light that seeps from a micro aperture with a size of 100 nm or less to perform on a resist local exposure that surpasses the limit of the wavelength of light.
However, any lithography apparatus that has the SNOM structure, which uses one (or more than one) processing probe to carry out fine processing has a problem that high throughput cannot be achieved.
U.S. Pat. No. 6,171,730 B1 (Japanese Patent Application Laid-Open No. H11-145051) discloses a method to solve this problem. According to the proposed method, a patterned photomask to let near-field light seep through a gap between shielding membranes is brought into close contact with a photoresist on a substrate, and then the photoresist is exposed to light, thereby transferring the whole minute pattern of the photomask onto the photoresist at the same time. The disclosed method is an excellent method and is of a great contribution to the technical field to which the present invention belongs.
The above-described near-field exposure method is capable of manufacturing a minute pattern of about several tens of nm, which is far smaller than the wavelength of exposure light. Accordingly, the above specification proposes to form a membrane portion in a photomask and to use pressure to let the membrane portion sag and to approach a photoresist to the near-field exposure region before the photoresist is exposed to light.
A description is given of light exposure that uses a photomask structured as shown in FIGS. 11A and 11B. In FIG. 11A, the photomask has a membrane portion 104, which is composed of a shielding membrane 102 and a membrane parent material 103. The shielding membrane 102 has a light exposure pattern 101.
In order to take full advantage of near-field exposure capable of manufacturing a minute structure beyond the limit of diffraction of light, it is necessary that the light exposure pattern 101 and a photoresist 107 approach each other to be in a near-field region (the distance between the two should be 100 nm or less, although it varies depending on the size of the light exposure pattern) during exposure. The membrane portion 104, therefore, sags upon exposure, so that the light exposure pattern is closely fitted to the photoresist (FIG. 11B).
The membrane portion that is brought into close contact with the resist is peeled off of the resist after exposure is completed. Under certain conditions, the membrane portion could be broken through repetitive fitting and peeling, thus making the photomask useless. When the membrane portion is torn, it mostly takes place at a border portion 106, which is the border between the membrane portion 104 and a substrate 105. This is probably because the stress caused by sagging of the membrane portion 104 locally concentrates on the border portion 106 between the substrate 105 and the membrane portion 104.
FIG. 10 shows a simulation of how the stress caused by sagging of the mask works on the membrane portion. In FIG. 10, the axis of the abscissa indicates the distance from the center of the membrane portion and the axis of the ordinate indicates tensile stress generated in the membrane portion. The membrane portion is 10 mm in diameter and the calculations are made accordingly.
As is clear from FIG. 10 and FIGS. 11A and 11B, the force applied to the membrane portion concentrates on the border portion 106 shown in FIGS. 11A and 11B, and the magnitude, as well as degree of concentration, of the force applied to the membrane portion is increased as the area where the photomask is in close contact with the photoresist is expanded. This means that the more the contact area is expanded to increase the light exposure pattern in size, the less durable the membrane portion becomes.
The magnitude of the force applied to the membrane portion is also dependent on the distance between the resist and the mask before the mask sags, and a larger force is applied to the membrane portion as the mask-resist distance is increased. In order to reduce the force applied to the membrane portion and to improve the durability of the photomask, the mask-resist distance should be set to be small. A fine, vertically-movable stage is necessary to control a small distance with precision. For instance, when exposing a photoresist to light by the step-and-repeat method in a stepper exposure apparatus, the position of the photoresist relative to a photomask is changed in a short period of time during exposure and it is, therefore, undesirable to raise the positional precision excessively, since it takes time. In addition, this makes the stage costly. As has been described, it is difficult to raise the precision of the mask-resist distance, in other words, to reduce the distance between the resist and the mask before the mask sags.